Employee Benefit Plans - Components of Net Period Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 71	$ 202	$ 255	$ 603
Interest cost	692	582	2,063	2,071
Expected return on plan assets	(889)	(777)	(2,686)	(2,328)
Amortization of net loss	614	434	1,592	1,556
Amortization of prior service cost	4	53	23	159
Effect of settlement		324		718
Net pension benefit cost	492	818	1,247	2,779
Pension | Retirement Income Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	8	16	23	49
Interest cost	354	231	1,063	1,022
Expected return on plan assets	(512)	(439)	(1,535)	(1,317)
Amortization of net loss	311	290	933	1,064
Amortization of prior service cost	0	0	0	0
Effect of settlement		265		540
Net pension benefit cost	161	363	484	1,358
Pension | National Service-Related Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	20	126	59	378
Interest cost	300	314	899	942
Expected return on plan assets	(342)	(294)	(1,027)	(881)
Amortization of net loss	179	144	536	492
Amortization of prior service cost	0	53	0	159
Effect of settlement		59		178
Net pension benefit cost	157	402	467	1,268
Pension | Superannuation
Defined Benefit Plan Disclosure [Line Items]
Service cost	43	60	173	176
Interest cost	33	31	86	90
Expected return on plan assets	(35)	(44)	(124)	(130)
Amortization of net loss	124	0	123	0
Amortization of prior service cost	4	0	23	0
Effect of settlement		0		0
Net pension benefit cost	169	47	281	136
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	5	6	15	17
Expected return on plan assets	0	0	0	0
Amortization of net loss	0	0	0	0
Amortization of prior service cost	0	0	0	0
Effect of settlement		0		0
Net pension benefit cost	$ 5	$ 6	$ 15	$ 17